Metal-organic frameworks for analytical chemistry: from sample collection to chromatographic separation

Isoreticular bio-MOF 100–102 coated solid-phase microextraction fibers for fast and sensitive determination of organic pollutants by the pore structure dominated mechanism

The stepwise ligand exchange strategy was successfully utilized for the improvement of adsorption ability of a series of bio-MOFs SPME fibers.

Magnetic Microparticles@UiO-67 Core-Shell Composites as a Novel Stationary Phase for High Performance Liquid Chromatography

Metal-organic framework UiO-67 was explored as a novel stationary phase for high performance liquid chromatography (HPLC). UiO-67 was, for the first time, homogeneously coated on carboxyl functionalized magnetic silica microparticles at low temperature (45 °C) by using a recently introduced liquid phase epitaxy (LPE) process. HPLC runs using the synthesized core-shell microparticles as stationary phase showed baseline separation for three phenol derivatives, applying gradient elution using acetonitrile and water as mobile phase. It also showed that UiO-67 has the largest affinity for 2,6-dichlorophenol among the phenol derivatives tested. The comparison of core-shell microparticles with 20 and 55 layers, respectively, of UiO-67 grown on the magnetic silica core proof that the UiO-67 shell determines separation behavior. Therefore, the use of UiO-67 core-shell microparticles as a stationary phase combines the advantages of a thin, homogenous MOF shell showing fast kinetics and good separation efficiency with the advantages of spherical silica microparticle cores offering high mechanical robustness and moderate pressure drop.

Heterogeneous nano metal-organic framework fluorescence probe for highly selective and sensitive detection of hydrogen sulfide in living cells

Hydrogen sulfide (H2S) has been regarded as the third important gaseous signaling molecule involved in human physiological and pathological processes. Due to the high reactive and diffusible properties of H2S, real-time detection of H2S fluctuations in living biological specimens is crucial. Here, we present a Cu(II)-metalated 3D porous nanoscale metal-organic framework (nano-MOF) {CuL[AlOH]2}n (PAC; H6L = meso-tetrakis(4-carboxylphenyl)porphyrin) and successfully employ this nano-MOF as a novel heterogeneous fluorescence probe for H2S detection. As far as we know, nano-MOFs have never been used as selective fluorescence probes for H2S detection. On the basis of the advantages of nano-MOF materials, this biocompatible nano-MOF probe exhibits rapid response, excellent selectivity, and hypotoxicity in in situ detection of H2S and represents the most sensitive fluorescence probe for selective H2S detection under physiological pH. In addition, confocal imaging was achieved successfully in living cells.

Capillary electrochromatographic fast enantioseparation based on a chiral metal-organic framework

Metal-organic frameworks (MOFs) have received great attention because of their unusual properties and fascinating structures in separation sciences. However, to the best of our knowledge, there has been no attempt to utilize chiral MOFs as stationary phases in packed-CEC. Here, a chiral MOF [In3 O(obb)3 (HCO2 )(H2 O)]·solvent (4,4'-oxybisbenzoic acid) was explored as the chiral stationary phase in packed-CEC for separation of chiral compounds and isomers. The fabricated [In3 O(obb)3 (HCO2 )(H2 O)]·solvent packed capillary columns gave fast enantioseparation of (±)-hydrobenzoin, (±)-1-phenyl-1,2-ethanediol, and clenbuterol within 3 min in CEC. Besides, the baseline separations of nitrophenol isomers within 6 min were also achieved. The RSDs for the retention time of run-to-run, day-to-day, and column-to-column reproducibility were 1.51-3.63, 1.83-3.98, and 3.42-5.66%, respectively. These results demonstrate that chiral MOFs are promising for enantioseparation in CEC.

Crystallography of metal-organic frameworks

Metal-organic frameworks (MOFs) are one of the most intensely studied material types in recent times. Their networks, resulting from the formation of strong bonds between inorganic and organic building units, offer unparalled chemical diversity and pore environments of growing complexity. Therefore, advances in single-crystal X-ray diffraction equipment and techniques are required to characterize materials with increasingly larger surface areas, and more complex linkers. In addition, whilst structure solution from powder diffraction data is possible, the area is much less populated and we detail the current efforts going on here. We also review the growing number of reports on diffraction under non-ambient conditions, including the response of MOF structures to very high pressures. Such experiments are important due to the expected presence of stresses in proposed applications of MOFs - evidence suggesting rich and complex behaviour. Given the entwined and inseparable nature of their structure, properties and applications, it is essential that the field of structural elucidation is able to continue growing and advancing, so as not to provide a rate-limiting step on characterization of their properties and incorporation into devices and applications. This review has been prepared with this in mind.

Separations of substituted benzenes and polycyclic aromatic hydrocarbons using normal- and reverse-phase high performance liquid chromatography with UiO-66 as the stationary phase

Metal-organic frameworks (MOFs) have great potential for applications in chromatography due to their highly tailorable porous structures and unique properties. In this work, the stable MOF UiO-66 was evaluated as both a normal-phase (NP-) and a reverse-phase (RP-) stationary phase in the high performance liquid chromatography (HPLC) to separate substituted benzenes (SBs) and polycyclic aromatic hydrocarbons (PAHs). It was found that the mobile phase composition has a significant effect on the HPLC separation. Baseline RP-HPLC separations of xylene isomers; naphthalene and anthracene; naphthalene and chrysene; and naphthalene, fluorene, and chrysene were achieved using MeOH/H2O ratios of 80:20, 75:25, 85:15, and 75:25, respectively, on the UiO-66 column. Similarly, baseline NP-HPLC separations of xylene isomers and ethylbenzene; ethylbenzene, styrene, o-xylene, and m-xylene; and several PAHs were also obtained on the UiO-66 column with different mobile phase compositions. The relative standard deviations (RSDs) of retention time, peak height, peak area, and half peak width for five replicate separations of the tested analytes were within the ranges 0.2-0.4%, 0.2-1.6%, 0.7-3.9%, 0.4-1.1%, respectively. We also evaluated other critical HPLC parameters, including injected sample mass, column temperature, and the thermodynamic characters of both the RP-HPLC and the NP-HPLC separation processes. It was confirmed that the separation of SBs on a UiO-66 column was an exothermic process, controlled by both enthalpy change (ΔH) and entropy change (ΔS). The reverse shape selectivity, size selectivity, stacking effect, and electrostatic force played vital roles in the separations of these analytes. To the best of our knowledge, this method is one of the very few examples of using MOFs as the stationary phase in both NP-HPLC and RP-HPLC. MOF-based stationary phases may thus be applied in the separations and analyses of SBs and PAHs in environmental samples.

Ultrathin-shell boron nitride hollow spheres as sorbent for dispersive solid-phase extraction of polychlorinated biphenyls from environmental water samples

Boron nitride hollow spheres with ultrathin-shells were synthesized and used as sorbents for dispersive solid-phase extraction of aromatic pollutants at trace levels from environmental water samples. Polychlorinated biphenyls (PCBs) were selected as target compounds. Sample quantification and detection were performed by gas chromatography-tandem mass spectrometry. Extraction parameters influencing the extraction efficiency were optimized through response surface methodology using the Box-Behnken design. The proposed method achieved good linearity within the concentration range of 0.15-250 ng L(-1) PCBs, low limits of detection (0.04-0.09 ng L(-1), S/N=3:1), good repeatability of the extractions (relative standard deviation, <12%, n=6), and satisfactory recoveries between 84.9% and 101.0% under optimal conditions. Real environmental samples collected from rivers, local lakes, rain and spring waters were analyzed using the developed method. Results demonstrated that the hexagonal boron nitride-based material has significant potential as a sorbent for organic pollutant extraction from environmental water samples.

A Novel Extraction-Cleanup Method for Determination of Trace PPCPs in Water Samples

In this contribution, a simple and sensitive extraction-cleanup method which was termed MAE-μ-SPE, was developed for the analysis of pharmaceutical and personal care products (PPCPs) in environmental water samples. The PPCPs included bisphenol A (BPA), diethyl phthalate (DEP), dibutyl phthalate (DBP), di (2-ethylhexyl) phthalate (DEHP), tetracycline (TC), deoxytetracycline (DC), oxytetracycline (OTC) and chloroteracycline (CTC). In this method, the PPCPs in the samples were extracted by microwave-assisted extraction (MAE) following adsorbed by copper (II) isonicotinate in micro-solid phase extraction (μ-SPE) device. The PPCPs were determined by high performance liquid chromatography with ultra-violet detector (HPLC-UV). The procedure of the MAE-μ-SPE was optimized in extraction temperature, extraction time, desorption time and desorption solvent. Analytical performances, such as limits of detection (in the range of 2.0-8.5 μg/L), quantification (in the range of 6.6-28.0 μg/L), and repeatability of the over-all procedure (less than 13%) were established. DEP, DBP, DEHP and TC studied in water samples were ranged from 18.2-68.8 μg L-1, while BPA, OTC, CTC and DC were found all below the detection limit in these samples.

Applications of homochiral metal-organic frameworks in enantioselective adsorption and chromatography separation

Chiral separation is of great importance for drug development, pharmacology, and biology. Chiral metal-organic frameworks (MOFs) is a new class of porous solid materials with high surface area, large pore size, high chemical stability, uniformly structured cavities, and the availability of modification. The excellent properties of MOFs have attracted intense interest to explore their performance and mechanism in chiral separation. This review summarizes three synthetic strategies of chiral MOFs and their applications in enantioselective adsorption and chromatographic separation. All the experimental and molecular simulation results demonstrated that high enantioselectivity was strongly correlated with a close match between the size of the pore and chiral molecules.

Recent developments in antibody-based assays for the detection of bacterial toxins

Considering the urgent demand for rapid and accurate determination of bacterial toxins and the recent promising developments in nanotechnology and microfluidics, this review summarizes new achievements of the past five years. Firstly, bacterial toxins will be categorized according to their antibody binding properties into low and high molecular weight compounds. Secondly, the types of antibodies and new techniques for producing antibodies are discussed, including poly- and mono-clonal antibodies, single-chain variable fragments (scFv), as well as heavy-chain and recombinant antibodies. Thirdly, the use of different nanomaterials, such as gold nanoparticles (AuNPs), magnetic nanoparticles (MNPs), quantum dots (QDs) and carbon nanomaterials (graphene and carbon nanotube), for labeling antibodies and toxins or for readout techniques will be summarized. Fourthly, microscale analysis or minimized devices, for example microfluidics or lab-on-a-chip (LOC), which have attracted increasing attention in combination with immunoassays for the robust detection or point-of-care testing (POCT), will be reviewed. Finally, some new materials and analytical strategies, which might be promising for analyzing toxins in the near future, will be shortly introduced.

Selective anion exchange with nanogated isoreticular positive metal-organic frameworks

Crystalline porous materials, especially inorganic porous solids such as zeolites, usually have negative frameworks with extra-framework mobile cations and are widely used for cation exchange. It is highly desirable to develop new materials with positive frameworks for selective anion exchange and separation or storage and delivery. Recent advances in metal-organic framework synthesis have created new opportunities in this direction. Here we report the synthesis of a series of positive indium metal-organic frameworks and their utilization as a platform for the anion exchange-based separation process. This process is capable of size- or charge-selective ion-exchange of organic dyes and may form the basis for size-selective ion chromatography. Ion-exchange dynamics of a series of organic dyes and their selective encapsulation and release are also studied, highlighting the advantages of metal-organic framework compositions for designing host materials tailored for applications in anion separation and purification.

Fabrication of metal-organic framework MIL-88B films on stainless steel fibers for solid-phase microextraction of polychlorinated biphenyls

Metal-organic frameworks (MOFs) have received considerable attention as novel sorbents for sample preparation due to their fascinating structures and functionalities such as large surface area, good thermal stability, and uniform structured nanoscale cavities. Here, we report the application of a thermal and solvent stable MOF MIL-88B with nanosized bipyramidal cages and large surface area for solid-phase microextraction (SPME) of polychlorinated biphenyls (PCBs). Novel MIL-88B coated fiber was fabricated via an in situ hydrothermal growth of MIL-88B film on etched stainless steel fiber. The MIL-88B coated fiber gave large enhancement factors (757-2243), low detection limits (0.45-1.32ngL(-1)), and good linearity (5-200ngL(-1)) for PCBs. The relative standard deviation (RSD) for six replicate extractions of PCBs at 100ngL(-1) on MIL-88B coated fiber ranged from 4.2% to 8.7%. The recoveries for spiked PCBs (10ngL(-1)) in water and soil samples were in the range of 79.7-103.2%. Besides, the MIL-88B coated fiber was stable enough for 150 extraction cycles without significant loss of extraction efficiency. The developed method was successfully applied to the determination of PCBs in water samples and soil samples.

A dual model logic gate for mercury and iodide ions sensing based on metal–organic framework MIL-101

In this work, by the introduction of metal–organic framework MIL-101 as the fluorescence anisotropy (FA) amplification and the low background platform, we firstly employed FA as the output signal of a logic gate, and constructed a dual model INHIBIT logic gate for Hg²⁺ and I⁻ detection.